Powder metallurgy is a processing technique whereby very small diameter powder particles are compressed into parts or shapes by a number of methods that include vacuum hot pressing, hot isostatic pressing (HIP), sinter hipping, hot forging, etc. These processes require the sequential or simultaneous application of high temperature and pressure. Typically, the temperatures used in powder metallurgy are at an appreciable fraction of the melting point (T.sub.m) of the compressed elements or alloys, usually above 0.8 T.sub.m. The pressures applied are often near or beyond the yield point of the metals involved. In the case of metal foils or sheets, consolidation is often done by hot roll bonding. One of the reasons for these severe conditions results from the need to break up the naturally occurring oxide on the surface of the material, thereby enabling the surfaces of the powder particles (foils or sheets) to weld together at a sufficient number of contact points so as to provide adequate adhesion between the individual particles, sheets or foils.
Powder metallurgy is useful as an alternative to comelting appropriate amounts of metal constituent components in forming intermetallic compounds. Intermetallic compounds have a great potential for a variety of applications as a result of their specific properties such as hardness, high elastic moduli and oxidation resistance. On the other hand, the inherent brittleness of intermetallic compounds severely curtails their use in conventional thermomechanical processing operations to form net shapes. As an alternative, powder technology is often used for processing intermetallic compounds. The starting materials for this approach are pre-alloyed compounds that have been comminuted by various methods into powder particles. The limitation of this approach is that it relies on compaction of powders which are inherently brittle and do not deform with ease. Compound formation, however, may also take place by solid state interdiffusion of mostly ductile constituent elements which can be compacted with relative ease. Mixtures of elemental metal powders, maintained in close mutual contact for a sufficient length of time at the appropriate temperature, interdiffuse and form intermetallic compounds. In some situations, intermetallic compound formation is required, but exposure to elevated temperature has to be limited or avoided. An example of such a situation may be the requirement for compound formation (for protheses or as dental restorative material) in a human body environment.
Intermetallic compound formation by interdiffusion of the constituent elements or extremely finely divided multi-phase solid formation by non-compound forming and non-interdiffusing elements is favored when the starting materials are in the form of a very small size particle powder. Such powders possess a large specific surface area, and hence, when mixed, form a relatively large interface area between the different constituents. The generation of an interface area between the different constituents depends on the efficiency of the mixing technique and also on the nature and properties of the mixed powder particles.
Several mixing techniques are commonly used in order to maximize the contact points (interface area) between particles of different kinds. If the effect of particle properties on the outcome of the mixing process is neglected, prolonged mixing will tend to maximize the number of contact points between different particles by striving towards a random distribution of the particles of different kind. Many particle properties such as particle size and shape, surface roughness, in addition to electrostatic phenomena, promote segregation effects and thus reduce and curtail the homogeneous mixing of different powders. Thus, multimodal particle size distribution favors space filling and increased density but also favors particle segregation. The most commonly used mixing technique relies on the tumbling-type blending of the powders. Ball milling is another technique that is used for mixing and also for reducing particle size. An extension of ball-milling is the mechanical alloying technique that yields alloyed powder products from elemental powder mixtures. Alloy or compound formation by ball milling is dependent on the kinetic energy input due to the rapidly rotating hard balls impinging on the powder particles. Thus ball-milling leads to high local temperature increases.
Intermetallic compound formation at the interface of two metals in intimate contact is a documented phenomenon. In some instances, the formation of intermetallic compounds is beneficial, in others, its effects may be detrimental. The formation of a new compound at temperatures below the melting point of the metals in contact relies on interdiffusion effects in the solid state. In most binary combinations, ambient temperature is well below the melting temperature of the constituent metals and, consequently, little or no compound formation takes place at the interface. Notable exceptions to this are diffusion couples in which one of the constituent metals, e.g. mercury or gallium, has a low melting point, below or close to room temperature. Another important group of binary combinations which shows room temperature compound formation, consists of a group I-B of the periodic table (Cu, Ag or Au) metal juxtaposed to a group III-A or IV-A (In, Sn or Pb) element. K. N. Tu et al., Jap. J. Appl. Phys. Suppl., Pt.1, 633 (1974). It is believed that room temperature compound formation in these systems is related to fast diffusion behavior of the noble or near noble component (the I-B elements) in the matrices of the group III-A or IV-A metals. A. D. LeClaire, J. Nucl. Mat. 69 & 70, 70 (1978). Fast diffusion occurs by virtue of the interstitial or partly interstitial diffusivity of the fast diffusing components, W. K. Warburton et al., "Diffusion in Solids, Recent Developments", Nowick and Burton (eds.), Academic Press, New York, 1975, p.172. It is noteworthy that interfaces between two components, each of which respectively belongs to one ot the two classes previously defined, are of common occurrence in electronic devices and it is not surprising, therefore, that such systems have been subject to relatively close scrutiny over the past years K. N. Tu, Ann. Rev. Mater. Sci., 15, 147 (1985). The quasi-totality of the room temperature intermetallic compound formation studies in these systems has made use of the thin film configuration. This configuration yields samples with a high interface to total volume ratio permitting effective study of compound formation at the interface. The phase diagrams in most binary combinations of this kind show the presence at room temperature of several equilibrium intermetallic compounds, (Table I).
TABLE I ______________________________________ Number of intermetallic compounds that are present in binary systems containing noble metals in which room temperature compound formation takes place. In Sn Pb ______________________________________ Cu 3 2 0 Ag 3 2 0 Au 4 4 2 ______________________________________
TABLE II ______________________________________ Numberof intermetallic compounds that are present in binary systems other than those containing noble metals in which fast diffusion effects take place. Ti Zr Gd.sup.1 U.sup.2 ______________________________________ Fe 2 4 4 2 Co 5 5 7 6 Ni 3 8 7 6 Pd 8(10) 4 6 5(7) Pt 4(6) 3 8 4 ______________________________________ .sup.1 A Gd matrix is taken as a prototype for lanthanide elements, as on of the two components of a binary combination. .sup. 2 Uranium matrix is taken as a prototype for other actinide elements, as one of the two components of a binary combination. .sup.3 In parenthesis, the number of intermetallic compounds including those stable at elevated temperatures or not yet well established.
Fast diffusion effects are not restricted to the above-mentioned systems. Other notable and relevant systems are combinations of an early transition metal element from the Group III-B (Sc, Y or a lanthanide element, Th and U) or from Group IV-B, (Ti, Zr or Hf) with a late transition metal from Group VIII (Fe, Co, Ni, Pd or Pt). Fast diffusion of the small size late transition metal elements in the matrix of the early transition metal elements has been reported in the literature. In these latter systems, however, none of the constituent elements has a melting point even relatively close to room temperature. Thus, in spite of fast interdiffusion, some exposure to intermediate temperatures is necessary in order to achieve any significant intermetallic compound formation within a reasonable time frame.
The formation of intermetallic compounds in even a relatively simple system such as two juxtaposed thin films, is a complex process. It depends on a number of variables such as the relative thickness of the individual initial layers, the diffusion mechanisms and the diffusivities of the atomic species in the different layers being formed, the nucleation characteristics of the various compounds, to mention just a few of the relevant parameters. It is not surprising, therefore, that in spite of the relatively large number of completed studies, no clear picture emerges regarding the outcome of the interdiffusion process in a thin film couple.
The thin film configuration, even though allowing an increase in the relative amount of compound to be formed at the interface, does not lend itself to the formation of bulk intermetallic compounds. Bulk formation of intermetallic compounds may be of both theoretical as well as practical interest. Bulk formation at room temperature and ambient pressure is of interest if extraneous constraints preclude the use of conventional processing and production methods, i.e. casting from the melt or diffusion assisted formation at elevated temperatures.
Another important use for powder metallurgy is its use in amalgams and related alloys. Metallic dental restorative materials used in dental fillings, placed directly in tooth cavity preparations, can be classified broadly into two classes, direct gold fillings and dental amalgams (O'Brien, 1989; Phillips, 1991). Dental amalgams are metallic composites resulting from a reaction between mercury and various pre-alloyed silver-tin-copper alloys. The mixing of mercury, which is liquid at ambient temperature, with the alloy in powder form takes place immediately prior to insertion in the dental cavity. The mixture, compacted into the cavity with dental instruments, consolidates into a cohesive solid and hardens over a length of time. Dental amalgams are much harder than pure gold fillings, they display relatively high compressive strength but are brittle and possess low transverse-rupture strength.
Amalgams and related alloys have been incorporated into a variety of commercial applications and thus a number of processes for producing such amalgams are known. For example, U.S. Pat. No. 4,664,855 discloses a universally employed process that triturates elemental metals or intermetallic alloys, in the form of comminuted filings or atomized spherical powders, with the sintering agent mercury and compacts the resulting amalgam into a uniform, consolidated metallic composite. The process may be considered a combination of liquid phase and reactive metal sintering. The finely comminuted metallic or intermetallic powders react with the Hg and when pressure is applied to the reaction product, form a compact, high density mass. U.S. Pat. No. 3,933,961 discloses a process for preparing a pre-weighed alloy tablet of uniform weight that is then triturated with a weighed quantity of Hg to form a traditional amalgam alloy.
The mercury content of dental amalgams has been a recurring source of concern because of the health and environmental hazards associated with its presence. Many aspects involved in the use of dental amalgams such as the various hazards, the possible substitute materials, their advantages and drawbacks, the economical considerations that are involved have been reviewed and discussed extensively in various publications as for example: Effects and Side-effects of Dental Restorative Materials", Adv. in Dental Res. 6:, September 1992; JADA Vol. 122, August 1991, papers p.54-61, p.63-65, p.67-71, p.73-77; JADA Vol. 125. April 1994, papers p. 381-387, p. 392-399).
Gold, either in the form of foils, powder or pellets can be used instead of mercury containing dental amalgams in direct filling. Prior to its condensation, pure gold, in all its forms, has to undergo a degassing procedure to desorb any adsorbed layers that might impede or prevent consolidation into a cohesive solid. Degassing is achieved by exposing the filling material to elevated temperature just before its insertion in the dental cavity. Clean gold surfaces and other noble metal surfaces, devoid of adsorbed layers, cold-weld under moderate pressure to form cohesive solids. Pure gold fillings are malleable and ductile and display high values of transverse rupture strength but low values of hardness and compressive strength.